Hipparcos sees the shade of a planet

1 January 2000

Between October 1989 and March 1993, the Hipparcos satellite of the European Space Agency measured the distances and the motions of more than 118 000 stars. Researchers from Observatoire de Paris took an active part in the reduction and the publication of the results of Hipparcos.

Nothing laid out this satellite to contribute to the study of extrasolar planets because the variation of the position of a star, due to the gravitational influence of a planet orbiting around it, is well too weak for direct measurements of position even for Hipparcos. But it was without counting the measurements of the star luminosity, carried out by the satellite jointly with the position measurements.

In 1995, the first extrasolar planet (i.e. revolving around another star that Sun) was discovered by M. Mayor & D. Queloz (Observatory of Geneva). Since then, planets were highlighted around 28 stars. In all cases, the same method was used: to measure, thanks to the periodic displacement of the lines in the spectrum of the star, the variations of its radial speed along its motion around the centre of mass of the system star+planet. Another method had been proposed: to seek the temporary attenuation of the stellar flux caused by the passage of the planet in front of the star. It is a transit, i.e. a partial eclipse.

A few weeks ago, two international teams discover, by the method of radial velocities, a news exoplanet around star HD 209458, in the constellation of Pegasus. As of November 7, 1999, and for the first time, this discovery is confirmed by the method of transits, also by two groups. The stellar flux decreases by approximately 2% during the partial eclipse (D. Charbonneau, Boulder University; G.W. Henry, Tennessee State University).

By examining the files of the Hipparcos Catalog, Noël Robichon and Frederic Arenou (Observatoire de Paris) showed that the satellite had observed 5 times the fall of luminosity due to passages of a planet in front of the star HD 209458 (*). The oldest observations dates from April 17, 1991, i.e. 4 years before the discovery of the first extrasolar planet and more than 8 years before the discovery of the HD 209458 planet transit by Charbonneau and Henry.

The planet, a gaseous giant, is so close to its star, that it carries out a revolution every 3,52 days. Thanks to the 8 years passed since the observations of Hipparcos, an excellent precision (1 second, i.e. 20 times better than the result by Charbonneau) was obtained over this period and makes it possible to predict in an extremely precise way the next passages of the planet in front of its star.

The Observatoire de Paris takes part in addition in two projects of satellites, COROT of CNES, which could detect, by the method of the partial eclipses, planets similar to the Earth, and GAIA of ESA, which would detect, in a systematic way, Jupiters orbiting around tens of thousands of stars. (*) Circular 7323, 1/12/99, of the International Astronomical Union and January issue of of the newspaper Astronomy & Astrophysics